Select and draw conclusions from samples 11.4

Population Sample A subset of the population A group of people or objects you want information about Sample A subset of the population

Sampling Methods – ways to choose your sample In a self-selected sample, members of a population can volunteer to be in the sample In a systematic sample, a rule is used to select members of the population, such as every third person. In a convenience sample, easy-to-reach members of a population are selected, such as those in the first row. In a random sample, each member of the population has and equal chance of being selected.

Bias in sampling In order to draw accurate conclusions about a population from a sample, you should select an unbiased sample. An unbiased sample is representative of the population you want information about. A sample that overrepresents or underrepresents part of the population is a biased sample. A random sample is often the preferred method of choosing a sample because it is most likely to be representative of the population.

Margin of Error The margin of error gives a limit on how much the responses of the sample would differ from the responses of the population. For example, if 40% of the people in a poll prefer candidate A, and the margin of error is + 4%, then it is likely that between 36% and 44% of the entire population prefer candidate A.

Margin of error formula When a random sample of size n is taken from a large population, the margin of error is approximated by this formula: Margin of Error =